386 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The babingtonite is black in color and of a brilliant vitreous lustre. 

 It is, however, very subject to decay, and when the protecting calcite has 

 been removed and the babingtonite exposed to the weather it becomes 

 dull and gradually alters to limonite, of which complete pseudomorphs 

 are frequently found. The decay is most active along the incipient 

 cleavage cracks, and crystals which appear on the surface fresh and 

 sound are often found on being broken to be permeated throughout 

 by limonite films. This character proved to be so universal in the 

 larger crystals that all attempts to prepare plates from them for the 

 study of the optical properties were unsuccessful. 



Crystallography. 



Twenty of the smaller crystals were measured and the following forms 

 determined, those marked with an asterisk being new to the mineral ; 

 c (001), b (010), a (100), h (110), g (2T0), f (320), k *(1T0), u *(054), 

 o (011), v *(035), w *(025), s (Oil), d (101), x *(305), y *(205), 

 n*(T01), p*(lll), t*(112), i*(TI2). 



The faces of the prism zone are generally deeply striated parallel to 

 the prism axis, and the basal plane is often curved or faceted. It was 

 found too that even where all the faces were plane there was often 

 a lack of parallelism in opposite faces, especially those of the prism zone, 

 which made the accurate adjustment on the goniometer difficult ; and for 

 this reason many of the crystals were measured both by the two-circle 

 method and by adjusting one plane in polar position and determining the 

 interfacial angles of the others to it. Both groups of measurements 

 are presented in the tables which follow, and the wide range of values 

 for many of the angles gives a measure of the irregularities to which 

 the crystals are subject. 



The position here adopted for babingtonite is that of Dauber, and his 

 elements have been used for the calculation of the table of angles.* 



* The choice of this position rather than the one taken by Dana and Hintze, 

 which is designed to bring out the similarity in form between babingtonite and 

 the monoclinic pyroxenes, may be justified on several grounds. The dominant 

 habit of the crystals, as may be seen from the figures, is prismatic in the direction 

 of Dauber's prism zone, so that this is the natural position in which to orient them 

 and verj' much simplifies the adjustment and measurement. In Dana's position 

 this zone is made a pyramid zone. Furthermore Dauber's position gives much 

 simpler symbols for the forms, as may be seen in the accompanying table. The 

 position of Goldschmidt (Winkeltabellen) gives symbols as simple, but, like Dana's, 



